A Safety‐Engineered Organic–Inorganic Composite Separator Enabling Fire‐Extinguishing and Dendrite‐Free Lithium Metal Battery Operation

ABSTRACT The limited thermal stability, mechanical fragility, and flammability of polyolefin separators impose inherent constraints on the safety and lifespan of lithium metal batteries. Although surface‐coating strategies have been widely explored, single‐component coatings, for example, Al 2 O 3 or polyimide, are unable to simultaneously address dendrite suppression, anti‐thermal shrinkage, and flame retardant. Given these critical challenges, we report a design strategy of multifunctional organic‐inorganic composite separator: (a) an ammonium polyphosphate and poly(acrylic acid) cross‐linked polymer provides intrinsic fire‐retardant behavior and features an electrolyte‐philic characteristic that facilitates Li‐ion transport; (b) a rigid Al 2 O 3 network embedded in the polymer matrix resists thermal shrinkage and blocks dendrite penetration through synergistic SEI protection layers rich in Li 3 N and Li 3 PO 4 . Together, these synergistic functions enable Li||Li cells to cycle stably for 2000 h and deliver long‐term stability in LFP||Li full cells (88% capacity retention over 1000 cycles). Moreover, the average heat release rate per unit area (0.04°C min −1 cm −2 ) of this battery during thermal runaway is markedly suppressed compared to those (0.3°C min −1 cm −2 ) of batteries using polyolefin separators, pouch cells incorporating this separator consistently exhibit rapid self‐extinguishing behavior during repeated ignition tests. This integrated safety‐centric architecture offers a practical pathway towards intrinsically safe, long‐life lithium metal batteries.